Electromyography
Synonym(s): EMG
Introduction
- Electromyography (EMG) covers methods that allow objective investigation of peripheral nerve, neuromuscular junction and muscle functioning.
ℹ️This technique is rarely used in cattle practice but may be considered in valuable animals as a useful complement to neurologic examination. Much of the information in this article is extrapolated from companion animal work.
- “Peripheral nerves” includes nerve cell bodies in the brain stem, spinal cord and ganglions and nerve roots from which the peripheral nerve originates.
- Electromyographic examination is divided into several procedures. The most reliable and widely used in a clinical setting are:
- Detection electromyography.
- Maximum motor nerve conduction velocity measurement.
- Maximum sensory/mixed nerve conduction velocity measurement.
- Repetitive motor nerve stimulation.
- In addition, ventral roots can be specifically investigated through F-wave studies.
- In detection EMG, muscles are investigated, looking for spontaneous electrical activity.
- In motor nerve conduction velocity studies, repetitive stimulations and F-wave studies, motor nerves are electrically stimulated and resulting electrical activity in their target muscles, evaluated.
- In sensory nerve conduction studies, nerves are stimulated, and recordings obtained from the same nerves.
- Denervated muscle fibers undergo many biochemical and physiological changes including:
- Increase in sensitivity to acetylcholine.
- Drifts in membrane potential.
- These result in spontaneous potentials observed between 5-10 days after a nerve injury.
- Parts of muscle fibers isolated from the end plate by a muscle lesion can survive and behave like denervated muscle cells, exhibiting spontaneous electrical activity.
- In both cases, spontaneous activities can be recorded by detection EMG.
- Disease processes may affect nerve cell body or processes, myelin sheaths or both. As a rule, the larger a nerve fiber, the faster its conduction velocity. A larger fiber also has higher metabolic requirements and usually is more susceptible to insults. Disappearance of the largest fibers results in a slight maximum conduction velocity drop and a marked diminution of the amplitude of the resulting muscle potential.
- Myelin sheath damage induces dramatic conduction slowing and conduction blockade, both identifiable through nerve conduction studies.
Uses
- Diagnostic technique to investigate neuromuscular abnormalities alongside the neurological examination:
- May predict severity of lesion.
- May localize the lesion more or less precisely.
- May discriminate whether nerve fibers or myelin sheaths in a nerve are most affected.
- Differentiate between disuse and denervation muscle atrophy.
Advantages
- Highly sensitive.
- May evidence changes before they become clinically detectable.
- May assist with more accurate guides to prognosis.
Disadvantages
- Expensive equipment.
- Requires and experienced and trained operator.
- Method currently restricted to academic and referral institutions, but is performed in the field in horses.
- Technical challenges:
- High quality grounding of the recording apparatus due to high amplifier gains.
- Requires careful upkeep of electrodes and connecting wires.
- Requires knowledge for interpretation.
Alternative techniques
- No alternative for functional exploration of peripheral nervous and muscular systems although lesional information is best drawn from microscopic examination of nerve and muscle biopsies.
- Serum measurement of CK enzyme activities and antibodies against end plates may contribute to the diagnosis of myositis/myopathy and myasthenia gravis respectively.
Time required
Preparation
- Sedation.
- Anesthesia: 20 min.
Procedure
- 90-120 min needed for complete exploration, but procedure may be tailored to individual cases and may take less time.
Decision taking
Criteria for choosing test
- Confirmation of, and discrimination among, peripheral nerve, nerve-muscle junction or muscular abnormalities.
- When need to refine site (local vs general, proximal vs distal, etc) of neural lesion.
Risk assessment
- No specific risk associated with the method.
- The only risk is related to the need for sedation or anesthesia.
- Patients with neuromuscular weaknesses, ie laryngeal paralysis and megaesophagus may be predisposed to aspiration pneumonia Aspiration pneumonia.
Requirements
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Preparation
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Procedures
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Further Reading
Publications
Refereed papers
- Recent references from PubMed and VetMedResource.
- Bolcato M, Mariana Roccaro, Jacinto Joana G. P et al. (2022) Use of electrodiagnostics in the diagnosis and follow-up of brachial plexus syndrome in a calf. Vet Sci 9 (3), 136 MDPI.
Motor nerve conduction velocity
- van Nes J J (1986) An introduction to clinical neuromuscular electrophysiology. Vet Quart 8 (3), 233-239 PubMed.
- van Nes J J (1986) Clinical application of neuromuscular electrophysiology in the dog: a review. Vet Quart 8 (3), 240-250 PubMed.
- Farnbach G C (1980) Clinical electrophysiology in veterinary neurology. Part I: Electromyography Comp Contin Edu Pract Vet 11, 791-797 ResearchGate.
- Sims M H & Redding R W (1980) Maturation of nerve conduction velocity and the evoked muscle potential in the dog. Am J Vet Res 41 (8), 1247-1252 PubMed.
- Brown N O & Zaki F A (1979) Electrodiagnostic testing for evaluation of neuromuscular disorders in dogs and cats. J Am Vet Med Assoc 174 (1), 86-90 PubMed.
- Swallow J S & Griffiths I R (1977) Age related changes in the motor conduction velocity in dogs. Res Vet Sci 23 (1), 29-32 PubMed.
- Lee A F & Bowen J M (1975) Effect of tissue temperature on ulnar nerve conduction velocity in the dog. Am J Vet Res 36 (9), 1305-1307 PubMed.
- Lee A F & Bowen J M (1970) Evaluation of motor nerve conduction velocity in the dog. Am J Vet Res 31 (8), 1361-1366 PubMed.
Repetitive stimulation
- Gödde T, Jaggy A, Vandevelde M et al (1993) Evaluation of repetitive nerve stimulation in young dogs. J Small Anim Pract 34 (8), 393-398 VetMedResource.
- Malik R, Ho S & Church D B (1989) The normal response to motor nerve stimulation in dogs. J Small Anim Pract 30 (1), 20-26 VetMedResource.
Sensory/mixed nerve conduction velocity
- van Nes J J (1985) Sensory action potentials in the ulnar and radial nerves of the dogs: effect of stimulation site and voltage. Am J Vet Res 46 (5), 1155-1161 PubMed.
- Redding R W, Ingram J T & Colter S B (1982) Sensory nerve conduction velocity of cutaneous afferents of the radial, ulnar, peroneal, and tibial nerves of the dog: reference values. Am J Vet Res 43 (3), 517-21 PubMed.
- Holliday TA, Ealand B G & Weldon B S (1977) Sensory nerve conduction velocity: technical requirements and normal values for branches of the radial and ulnar nerves of the dog. Am J Vet Res 38 (10), 1543-1551 PubMed.
F-waves and H-reflexes
- Cuddon P A (1998) Electrophysiologic assessment of acute polyradiculoneuropathy in dogs: comparison with Guillain-Barre syndrome in people. J Vet Intern Med 12 (4), 294-303 PubMed.
- Poncelet L & Balligand M (1991) Nature of the late potentials and F-ratio values in dogs. Res Vet Sci 51 (1), 1-5 PubMed.
- Malik R & Ho S (1991) A new method for recording H-reflexes from the plantar muscles of dogs. J Small Anim Pract 32 (11), 547-556 VetMedResource.
- Steiss J E (1984) Linear regression to determine the relationship between F-wave latency and limb length in control dogs. Am J Vet Res 45 (12), 2649-2650 PubMed.